That depends. For example if using GV on your mobile phone:Is calling to us numbers free
BMaking domestic calls
When you use Google Voice on your mobile phone, domestic calls will use the standard minutes from your cell phone plan.
Each time you place a call, instead of dialing the number you want to reach, Google Voice dials a US-based direct access number. The Google Voice server then forwards your call to the actual destination number. This allows for faster call setup and the ability to place calls without a data connection. These direct access numbers may show in your phone's call log, or on your phone bill.
A unique direct access phone number is assigned to each person that you call. The direct access numbers are stored in the Google Voice application, not in your Contacts. They can be purged from your phone at any time by signing out of the Google Voice app.
Direct access numbers are assigned randomly, and may include numbers that are out of your local calling area, or even out of state. If you don't have a nationwide calling plan, you may be charged long distance rates by your carrier.